784 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



A board about 14 feet long, and 6 inches broad, fig. 7, pi. 14, has 2 slips of deal, 

 an inch square, fastened on the 2 sides: these make a groove, for 2 short pieces, 

 to slide in, backwards and forwards. The 2 sliding pieces, figs. 8 and 9, carry 

 each a small board or vane; one towards the right, the other towards the left; but 

 so as to meet in the middle, and apparently to make but one when placed side by 

 side. The vanes are covered with a piece of fair white paper, which is to reflect, 

 or rather to scatter light in every direction. To one end of the board is fixed a 

 circular piece of wood, with an opening in it, which is afterwards to be shut up 

 by a small moveable piece, fig. 10, intended for placing the transmitting objects 

 on. This moveable piece contains 2 holes, at the distance of 14- inch from centre 

 to centre, and a inch in diameter each. Against the circular wooden screen, and 

 close over the opening in it, is placed a lantern containing a lamp; fig. 11. Its 

 construction is such as to admit a current of air to feed the flame from below, by 

 means of a false bottom, and to let it out by a covered roof; and the whole of the 

 light, by the usual contrivance of dark lanterns, is thus kept within, so as to leave 

 the room in perfect darkness. In the front, that is towards the vanes, the lantern 

 has a sliding door of tin-plate, in which there is a parallelogrammic hole, covered 

 with a spout 5 inches long, of the same shape. Two or 3 such doors, with dif- 

 ferent spouts and openings, will be required to be put in, according to the ex- 

 periments to be made; but the first will do for most of them. 



A narrow arm is fastened to the long board, which advances about 3 feet beyond 

 the screen, and carries a circular piece of pasteboard, that has an adjustable hole 

 in the centre, through which the observer is to look when the experiments are to 

 be made. At the further end of the long board is a pulley, over which a string, 

 fastened to the back of the slider that carries one of the vanes, is made to pass. 

 This string returns under the bottom of the long board, towards the other end, 

 where, close to the observer, another pulley is fixed; and, after going also over 

 this pulley, it returns at the top of the board, to the front of the same vane, to 

 which the other end of it is fastened at the back. By pulling the string either 

 way, the observer may bring forward the moveable vane, or draw it back, at plea- 

 sure. At the side of the long board is a scale of tens of inches, numbered from 

 the place of the flame of the lamp, O, 10, 20, 30, 40, and so on to J 60. A 

 pair of compasses being applied from the last 10 towards the vane, ascertains its 

 distance from the flame, to as great an accuracy as may be required. 



When the transmitting power of a glass is to be tried, it must be placed over 

 one of the holes of the small moveable piece, which then is fastened with a button, 

 on the opening left for it in the circular wooden screen. Then, looking through 

 the hole of the pasteboard at the 2 vanes, and bringing that which is seen through 

 the glass near enough to give an image equally bright with that which is seen 

 through the open hole, the observation will be completed. Having measured the 

 odd inches by a pair of compasses, or immediately by a scale, we deduce, as usual, 

 the transmitting power, by taking double the logarithm of the distance of the 



